1. Field of Invention
The present invention generally relates generally to an illumination system of the type used in lithographic apparatus for semiconductor wafer manufacture.
2. Related Art
A lithographic apparatus applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically carried out by imaging the pattern using a UV radiation beam onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. Another lithographic system is an interferometric lithographic system where there is no patterning device, but rather a light beam is split into two beams, and the two beams are caused to interfere at a target portion of substrate through the use of a reflection system. The interference causes lines to be formed on at the target portion of the substrate.
Lithography apparatus may use an alignment system for detecting the position of alignment marks on a wafer and align the wafer using the alignment marks to ensure accurate exposure from a mask. Alignment systems typically have their own illumination source. The signal detected from the illuminated alignment marks can be affected by how well the illumination wavelengths are matched to the physical or optical characteristics of the alignment marks, or physical or optical characteristics of materials in contact with or adjacent to the alignment marks. The aforementioned characteristics can vary depending on the processing steps used. Phase-grating alignment systems commonly offer a set of discrete, relatively narrow band illumination wavelengths in order to maximize the quality and intensity of alignment mark signals detected by the alignment system. The specific discrete wavelengths are often limited to the types of sources commercially available.
While a selection of discrete wavelengths allows flexibility to choose a wavelength that improves the alignment signal for a given set of alignment mark and other local characteristics as described earlier, certain lithographic processes and/or alignment marks may require an illumination wavelength that falls outside the discrete wavelengths that are generated by conventional alignment systems. If the optimal narrow band of radiation required for a particular alignment mark or lithographic process falls in between, or outside of, a set of discrete set point options, the alignment performance will be adversely affected, perhaps to the level that alignment is not possible. This limitation reduces the flexibility to modify lithographic processes and/or alignment marks. Methods and systems are needed to overcome the above mentioned deficiencies.